Abstract

The sunglint geometrical optics equations of a statistically faceted sea, supported by the
so-called interaction probability density and employing an averaging hybrid of the Cox–Munk
and Mermelstein slope statistics, was successful in simulating 0.5 μm sunglint region
characteristics. The results match independent experimental data, and good agreement is reported
for various sea conditions and Sun locations, on sunglint amplitude, sunglint location, and
azimuth range. In particular, the peak reflectance shift from the specular direction toward the
horizon is correctly predicted, and it is found that the physical mechanism responsible for the
shift is the accumulation of contributing facets near the horizon.

Full simulated normalized sunglint contribution (reflectance) corresponding to a 59.3° SZA and a mean wind speed of 4.9 m∕s, and direction of 244.5°N. This was obtained by use of the average CM&M statistics and is found to compare very favorably with the measurements of Su et al. [Ref. 6, Fig. 6(a)].

Full simulated normalized sunglint contribution (reflectance) corresponding to a 58° SZA and a mean wind speed of 4.8 m∕s was obtained by use of the average CM&M statistics and is found to compare very favorably with the measurements of Su et al. [Ref. 6, Fig. 10(a)].

Normalized sunglint contribution (reflectance) in the central plane (azimuth ∼188°) corresponding to a 58° SZA and a Sun azimuth of ∼188°, with a mean wind speed of 0.6 m∕s. Our simulated data comprises (a) CM, (b) M, and (c) CM&M statistics, and is compared with measurements by Su et al. [Ref. 6; Fig 10(b)]. Although only five wind directions in one quadrant are considered explicitly, the data correspond to all quadrants in view of the symmetries about the downwind and crosswind directions.

Full simulated normalized sunglint contribution (reflectance) corresponding to a 58° SZA and a mean wind speed of 0.6 m∕s was obtained via use of the average CM&M statistics and is found to compare favorably with the measurements of Su et al. [Ref. 6, Fig. 10(b)].

Normalized sunglint contribution (reflectance) in the central plane corresponding to the Sun just above the horizon, with 83.1° SZA, a Sun azimuth of ∼128°, and a mean wind speed of 5.8 m∕s. Our simulated data comprises CM, M, and CM&M statistics, all with and without a transmittance multiplicative factor of 0.15. The simulated data are compared with measurement data read from Su et al. [Ref. 6, Fig. 5(a)], and decent agreement is observed for the case of CM&M with transmissivity and look-down angles larger than the measure of the Sun elevation above the horizon.

Qualitative description of reflectance. (a) Geometry involving a contributing (specular) facet at angle θ. The qualitative picture provided by Eq. (12) is represented in the reflectance curves (b), for CM statistics, several wind speed conditions, and Sun elevation at θ0 = 40° (the arrow denotes the specular angle). For a rough sea, and as θ → 0, the angular density of contributing facets increases significantly, leading to a shift of the reflectivity peak toward the horizon. The effect increases as the seas become rougher.